MicroRNAs (miRNAs) are a class of 22-nt noncoding RNAs, which are evolutionarily conserved and function as negative regulators of gene expression. Like conventional protein-coding mRNA, miRNAs are transcribed by RNA polymerase II, spliced and polyadenylated (called primitive miRNA or pri-miRNA). However, unlike mRNA, the pri-miRNAs contain a stem-loop structure that can be recognized and excised by the RNAi machinery to generate hairpin precursor miRNAs (pre-miRNA) that are ˜70 nt in animals or ˜100 nt in plants. Pre-miRNAs are cleaved by cytoplasmic RNase III Dicer into a ˜22-nucleotide miRNA duplex: one strand (miRNA*) of the short-lived duplex is degraded, whereas the other strand serves as a mature miRNA. The mature miRNA then guides a complex called miRNP (miRNA-containing ribonucleo-protein particles) to the complementary site(s) in the 3′ untranslated region (UTR) of a target mRNA. Consequently, translation blockade or mRNA degradation will occur depending on whether it is partially matched or completely matched with the target genes, respectively (1). Moreover, the levels of individual miRNAs are dramatically changed in different cell types and different developmental stages, suggesting that miRNA plays a role in cell growth, differentiation, and programmed cell death (1, 2). miRNAs have been shown to be aberrantly expressed or mutated in human cancer, indicating that they may function as a novel class of oncogenes or tumor suppressor genes (3-9). The first evidence of involvement of miRNAs in human cancer came from molecular studies characterizing the 13 q 14 deletion in human chronic lymphocytic leukemia, which revealed two miRNAs, miR-15a and miR-16-1 (3). Subsequently, miRNA deregulation was detected in other human malignancies, including breast carcinoma (4, 5), primary glioblastoma (6, 7), lung cancer (8), papillary thyroid carcinoma (9), colon carcinoma (10) and pancreatic tumors (11, 12). For instance, the miR-17-92 cluster is upregulated in B-celllymphomas and lung cancer. miR-143 and -145 are down-regulated in colon carcinomas. A decrease in Let-7 is detected in human lung carcinomas and restoration of its expression induces cell growth inhibition in lung cancer cells (13). The BIC gene, which contains the miR-155, is up-regulated in some Burkitt's lymphomas and several other types of lymphomas (14, 15).